Serometacoceras dorsoarmatum ( Abich, 1878 ), 2025

Korn, Dieter & Ghaderi, Abbas, 2025, Late Permian nautiloids from Julfa (NW Iran), European Journal of Taxonomy 1018, pp. 1-113 : 36-38

Serometacoceras dorsoarmatum ( Abich, 1878) gen. et comb. nov.

Figs 3C , 17 ; Table 9

Nautilus dorso armatus Abich, 1878: 20 , pl. 4 fig. 1.

Pleuronautilus dorso-armatus – von Arthaber 1900: 215.

Pleuronautilus ( Pleuronautilus) dorso-armatus – Kummel 1953: 36.

Metacoceras dorsoarmatum – Shimansky 1965a: 41, pl. 14 fig. 5. — Teichert & Kummel 1973: 417, pl. 3 figs. 5–6.

Diagnosis

Species of Serometacoceras gen. nov. with thickly discoidal, subevolute conch (ww/dm =0.45–0.50; uw/dm= 0.40–0.45), weakly depressed whorl profile (ww/wh = 1.15–1.40) and very high coiling rate (WER= 2.30–2.50) at a conch diameter of 50 mm. Whorl profile weakly trapezoidal with gently divergent flanks; venter and flanks flattened. Sculpture with 10–12 low ventrolateral nodes per volution; the nodes show a short extension toward the midflank. Suture line with a broad and shallow external lobe and a slightly deeper, broadly rounded lateral lobe.

Material examined

IRAN – East Azerbaijan • 1 specimen; Ali Bashi 4; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2018; Ghaderi leg.; illustrated in Fig. 17A–C ; MB.C.32026 . – West Azerbaijan • 1 specimen; Aras Valley ; Araxoceras Beds of the Julfa Formation ( early Wuchiapingian); 2018; Ghaderi leg.; illustrated in Fig. 17D–F ; MB.C.32027 • 1 specimen; same data as for preceding; 2012; Korn et al. leg.; illustrated in Fig. 17G ; MB.C.32028 .

Description

Specimen MB.C.32026 is the largest specimen available; it is a somewhat corroded fragment of an internal mould with a whorl height of 38 mm, corresponding to a conch diameter of about 115 mm ( Fig. 17A ). Only one third of the whorl is preserved, showing remains of the last three phragmocone chambers and a part of the body chamber. The whorl profile is trapezoidal and slightly depressed (ww/ wh = 1.19) with a broadly rounded venter and a prominent, narrowly rounded ventrolateral shoulder. The flattened flanks are weakly divergent and the umbilical margin is broadly rounded ( Fig. 17B ). The sculpture consists of very shallow ribs beginning on the inner flank and becoming coarser towards the outer flank where they terminate in transversely elongated, low and blunt ventrolateral nodes. They become much weaker in the last preserved part of the body chamber. It appears that these nodes do not coincide between the two sides of the conch, but rather alternate in position. The suture line of the specimen shows a very broad and flat external lobe, a broadly rounded ventrolateral saddle and a flat, broadly rounded lateral lobe ( Fig. 17C ).

Specimen MB.C.32027 ( Fig. 17D ) is an internal mould fragment of a specimen with a conch diameter of about 48 mm, consisting of a quarter of a whorl with the last phragmocone chambers and part of the body chamber. It has a subquadrate, weakly depressed whorl profile (ww/wh= 1.17) with a flatly rounded venter, a narrowly rounded ventrolateral shoulder, slightly concave and weakly divergent flanks and a raised and thus rather pronounced, but rounded umbilical margin ( Fig. 17E ). The ventrolateral shoulder bears short radially elongated nodes that extend only a short distance towards the flank, were they are visible only as very low ridges. The umbilical margin bears barely visible, very small tubercles. The suture line has a very shallow external lobe and a broadly rounded lateral lobe ( Fig. 17F ). The septa are separated by an average of about 13 degrees.

Specimen MB.C.32028 is externally corroded and was sectioned for the study of the inner whorls; it is slightly deformed and was digitally re-deformed for the illustration ( Fig. 17G ). The cross section shows some ontogenetic changes in the shape of the whorls, starting with a circular profile in the early ontogenetic stage, followed by an inverted trapezoidal shape at a conch diameter of 11.5 mm. Half a volution later, at 19 mm diameter, the whorl profile is rounded-hexagonal with a pronounced, slightly raised umbilical margin and parallel flanks. In the course of half a volution from a conch diameter of 30 to 45 mm, the flanks become clearly divergent and slightly concave. At 39 mm diameter, the section shows a pronounced ventrolateral node extending about 3 mm beyond the ventrolateral shoulder. Finally, the whorl profile of the last volution shows a broadly rounded umbilical margin and flanks that merge continuously into the umbilical wall.

Only minor ontogenetic changes in the cardinal conch parameters can be detected, but it must be taken into account that the ww/dm and ww/wh ratios can be influenced by the position of the cross section relative to the ribs. Whether the section hits a node or interspace is critical to the shape of the whorl profile. However, it can be seen clearly that the uw/dm ratio slowly increases from a value of 0.33 at a conch diameter of 11.5 mm to 0.42 at 68 mm diameter.

Remarks

Abich (1878: pl. 4 fig. 1) illustrated a specimen with a diameter of 53 mm under the name “ Nautilus dorso armatus , nov. form.”; this fragment allows a fairly accurate description of the morphology of the conch and the sculpture ( Fig. 3C ). The conch is subevolute (uw/dm ~0.40) with a weakly depressed whorl profile (ww/wh ~1.25). The venter appears to be flattened and the flanks are bordered by a rounded ventrolateral shoulder and a rounded but distinct umbilical margin. The sculpture consists of five laterally directed nodes on half a whorl, which are about half as wide as their interspaces. They are somewhat elongated towards the umbilicus and extend almost to the middle of the flank. The penultimate whorl bears about eight shallow radial riblets on the flank on half a volution. The suture line extends with shallow and rounded lobes across venter, flanks and umbilical wall.

Shimansky (1965b: pl. 14 fig. 5) showed a more complete specimen with a conch diameter of 72 mm; this agrees well with Abich’s original specimen. The close resemblance of these two specimens to the newly collected material from Iran allows a fairly good identification of the new specimens.

Serometacoceras dorsoarmatum gen. et comb. nov. is very similar to S. dorashamense gen. et comb. nov., but differs in having considerably shorter ribs, confined to the outer flank. In contrast to S. dorashamense , S. dorsoarmatum has no umbilical nodes. Another, albeit small, difference lies in the width of the umbilicus. In S. dorsoarmatum , the uw/dm ratio is greater than 0.40 and in S. dorashamense only around 0.37.

Figures

Fig. 3. Reproductions of nautiloid figures of Abich (1878). A. “Nautilus dorso plicatus” Abich, 1878. B. “Nautilus dorso plicatus”. C. “Nautilus dorso armatus” Abich, 1878. D. “Nautilus incertus” von Arthaber, 1900. E. “Nautilus Pichleri” von Arthaber, 1900. F. “Nautilus tubercularis” Abich, 1878. Scale bar units =1 mm.

Fig. 17. Serometacoceras dorsoarmatum (Abich, 1878) gen. et comb. nov. from the Araxoceras Beds of the Julfa Formation. A. Specimen MB.C.32026 (Ghaderi 2018 Coll.) from Ali Bashi 4, lateral view. B. The same specimen, whorl profile. C. The same specimen, suture line at ww = 45.0 mm, wh= 38.5 mm. D. Specimen MB.C.32027 (Ghaderi 2018 Coll.) from Aras Valley, lateral view. E. The same specimen, whorl profile. F. The same specimen, suture line at ww = 23.5 mm, wh =18.5 mm. G. Specimen MB.C.32028 (Korn et al. 2012 Coll.) from Aras Valley, cross section and reconstruction of apertural view. Scale bar units =1 mm.

Tables

Table 9. Conch dimensions (in mm) and ratios of Serometacoceras dorsoarmatum (Abich, 1878) gen. et comb. nov.; reconstructed dimensions and ratios in italics.

Nr.	dm	ww	wh	uw	ah	ww/dm	ww/wh	uw/dm	WER	IZR
MB.C.32026	115.0	45.4	39.0	47.0	37.0	0.39	1.16	0.41	2.17	0.05
MB.C.32027	48.2	22.8	19.5	19.4	18.0	0.47	1.17	0.40	2.55	0.08
MB.C.32028	67.9	29.8	23.6	28.8	22.7	0.44	1.26	0.42	2.26	0.03
MB.C.32028	45.2	22.6	15.6	18.4	15.3	0.50	1.45	0.41	2.29	0.01
MB.C.32028	29.8	15.3	11.2	11.0	10.8	0.51	1.37	0.37	2.45	0.04
MB.C.32028	19.1	9.4	7.7	6.4	7.4	0.49	1.23	0.34	2.66	0.04
MB.C.32028	11.7	5.9	5.0	3.8	5.0	0.51	1.18	0.33	3.06	0.00